FIELD OF THE INVENTION
The invention relates to a method for data transmission on transmission channels in a digital transmission system, in which, for channel coding, turbo coding is carried out in a turbo coder at a transmitter end and turbo decoding with soft-decision output signals is carried out in a turbo decoder at a receiver end.
The use of turbo codes for digital transmission systems is investigated in a reference by P. Jung, titled "Comparison of Turbo-Code Decoders Applied to Short Frame Transmission Systems", IEEE Journal on Selected Areas in Communications, Volume 14 (1996) pages 530-537, with both coders and decoders being investigated for the turbo codes in the transmission path. Decoding of the turbo codes is based on the use of soft-input/soft-output decoders, which can be produced using either maximum a-posteriori (MAP) symbol assessors or MAP sequence assessors, for example an assessor using an a-priori soft-output Viterbi algorithm (APRI-SOVA). This publication describes four different decoder configurations and their capabilities to process specific error rates. Furthermore, the performance of these decoders is investigated for different applications. It has been found that the turbo codes and their iterative decoding are an effective measure against packet errors.
The reference from ICC '95, Seattle, Wash., Jun. 18-22, 1995, titled "Turbo Codes for PCS Applications", D. Divsalar and F. Pollara, proposes turbo codes to achieve error correction virtually as far as the so-called Shannon limit. Relatively simple component codes and large interleavers are intended to be used for this purpose. In this publication, the turbo codes are produced in a coder using multiple codes, and are decoded in a suitable decoder. The turbo codes were introduced by a reference by Berrou et al. 1993 (see C. Berrou, A. Glavieux and P. Thitimayshima, titled "Near Shannon Limit Area Correction Coding: Turbo Codes"Proc. 1993 IEE International conference on communications, pages 1064-1070). On the one hand, this method allows very good error correction to be achieved. So-called turbo equalization is known from ETT European Transactions on Telecommunications, Vol. 6, No. 5, September-October 1995, titled "Iterative Correction of Intersymbol Interference: Turbo-Equalization", Catherine Douillard et al., whose use is intended to overcome the disadvantageous effects of intersymbol interference in digital transmission systems which are protected by convolution codes. The receiver makes two successive soft-output decisions, which are made in an iterative process by a symbol detector and a channel decoder. Each iteration makes use of extrinsic information from the detector and the decoder for the next iteration, as with turbo decoding. It has been found that intersymbol interference effects in multipath channels can be overcome by turbo equalization.
Future transmission systems, for example the European universal mobile telecommunications system (UMTS), require the support of a large number of co-existing carrier services with carrier data rates of up to 2 Mbit/s in a flexible manner, with the best-possible spectral efficiency being desirable. An multiple access (MA) scheme has been developed in the advanced communications technologies and services (ACTS) project AC090 FRAMES (Future Radio Wideband Multiple Access Systems), which is called FRAMES Multiple Access (FMA) and satisfies the UMTS requirements. As a third-generation transmission system, which covers a wide range of application areas, carrier services and widely differing scenarios, FMA must comply with present and future developments of UMTS radio interface standards. FMA contains two operating modes, namely wideband time division multiple access (WB-TDMA) with and without spreading and compatibility with global system for mobile communications (GSM) and wideband code division multiple access (WB-CDMA). Although, essentially, a system based on FMA is considered here, it is also possible to include other transmission systems using multiple access methods, for example frequency division multiple access (FDMA), MC-CDMA (Multicarrier-CDMA) or combinations of the transmission systems.
With regard to the high performance of turbo codes, it is desirable to use these in digital transmission systems. The complex requirements, for example for FMA, results, however, in that it is necessary when using such turbo codes to ensure that the data transmission makes full use of the capabilities of the turbo codes.
The performance of turbo coders and turbo decoders for channel coding using alternative coders and decoders was compared at the 5th International Symposium on Personal, Indoor And Mobile Radio Communications (PIMRC '94), and ICCC Regional Meeting on Wireless Computer Networks (WCN), The Hague, NL, Vol. 2, 18-23 September 1994, pages 524-528; M. Nasshn et al. It is stated that information relating to the system performance is obtained using the turbo decoder by determining the variance of the log-likelihood ratios.
The reference IEEE Transactions on Communications, New York, US, Vol. 43, No. 2/4, Part 2, February 1995, pages 733-737; by R. H. Deng et al., describes a Type I hybrid ARQ system which automatically matches the code rates to the bit error rates of the channel. Punctured convolution codes are used for transmission.
In Published, European Patent Application EP 0 755 122, an adaptive termination criterion is used for iterative decoding of transmitted, multidimensionally coded information. The relative entropy, approximations to the relative entropy or variables similar to it are used as a measure for the change in the weighted decisions for successive iteration step elements.